Citty match against PSG will determine if they can survive the group stage. If they fail to win PSG, they are gone. It's only the transfer window that can bring back hope to City because their current players have nothing to offer again for the club. EPL is the only league that City can be hopeful of and that depends on the team's strength after getting new players in January.
Manchester City is a team that has a lot of finances so I am also sure that the team can bring in some new players through the January transfer window. Because Pep and the team management must continue to move to see that option as early as possible in the transfer window because it is true as you said if Manchester City only hopes for a miracle from the players they currently have, of course there will be no significant change in performance for them this season. So the choice must still be able to get new players in order to find changes in their strengths so that they are not immediately eliminated from the UCL this season.
It looks like this will be a deciding match for both of them because whoever loses this match will have a tough time qualifying for the playoffs. They don't just have to win the last game, but they will also likely depend on the matches of other teams. If you look at the current situation, PSG might have a better chance of winning than Man City because they have the home advantage and Man City has been really bad recently. Luckily the match will still be held for quite some time, so Man City still has time to improve their performance, and they might take advantage of this January transfer window.